fix(registry): handle compound subgraph node IDs, add proactive node push from graph hooks

- Handle compound node IDs (e.g. "252:0") from expanded group subgraphs
  to fix 400 Bad Request on workflows with group nodes
- Frontend proactively pushes node data via afterConfigureGraph and
  LiteGraph hooks (onNodeAdded/onNodeRemoved/graphChanged), eliminating
  WebSocket round-trip latency for most "Send to Workflow" operations
- Add content-fingerprint dedup to skip duplicate register-nodes POSTs
- Fast-path cache returns immediately when tabs are registered (including
  0-node registrations), avoiding unnecessary WS refresh cycles
- Distinguish "Empty Registry" from other errors in standalone UI toast
- Reduce WS refresh timeout 2s→0.5s, add cooldown and lock to prevent
  concurrent refresh storms
- All [LM:Registry] logs at DEBUG level

static/js/utils/uiHelpers.js

@@ -552,6 +552,8 @@ async function fetchWorkflowRegistry() {
     if (!registryData.success) {
       if (registryData.error === 'Standalone Mode Active') {
         showToast('toast.general.cannotInteractStandalone', {}, 'warning');
+      } else if (registryData.error === 'Empty Registry') {
+        showToast('uiHelpers.workflow.noSupportedNodes', {}, 'warning');
       } else {
         showToast('toast.general.failedWorkflowInfo', {}, 'error');
       }